Field of the Invention
The invention relates to a method and to a device for producing a structured, at least partly optically transparent varnish surface on the surface of a substrate board, preferably of a wood material board, which has a decoration.
Description of Related Art
There are various methods for producing floor, wall or ceiling panels and furniture construction panels with decorative surfaces. In addition to methods in which decorative surfaces are produced on wood material boards by coating said boards with printed, resin-impregnated paper (decor paper), methods are also known for printing a decoration, in particular a wood decoration, directly onto wood material boards.
For example, WO 2006/002917 A2 by the Applicant describes a method for producing a wood material board having a decorated surface. In this known method, a surface of the wood material board, which has already been ground and is to be provided with a decoration, in particular with a wood decoration, is primed, then smoothed and subsequently printed directly in the gravure printing method by means of printing rollers (printing cylinders), to produce the decoration.
To produce panels with decorative surfaces, digital printing methods are also used in addition to roller printing methods. Thus for example, WO 98/26936 A1 discloses a method for producing resin-impregnated decor papers for coating substrate boards. In this method, a paper is printed by a digital printer, in particular by a laser printer. The decor paper is pressed onto the substrate board in a hot press, for example in a short-cycle press, the resin which has impregnated the paper being thereby cured. The substrate boards coated with the decor paper (laminates) are intended in particular for furniture construction.
To protect the decoration against abrasive wear, the decor paper or the decoration printed directly onto the substrate board is provided with a transparent sealing layer.
The sealing layer of the decorative surfaces produced thus is usually structured such that it has a three-dimensional surface structure in the end. The three-dimensional structure improves the haptic effect of the decorative surface. The haptic impression produced by the structured decorative surface thereby corresponds to the surface character to be imitated, for example a porous wood surface.
To produce structured surfaces on wood material boards provided with a decoration, structured pressing plates, structured pressing belts or embossing rolls are used in the prior art. Furthermore, structured varnish applicator rolls are also used for this purpose. The production of these plates, belts and rolls is associated with considerable costs. Furthermore, changing said plates, belts and rolls is very time-consuming, which is a disadvantage in the case of a single production plant in which wood material boards are to be produced with different decorations and correspondingly different surface structures.
To solve this problem, it has been proposed to produce the surface structures by directly applying varnish to the board-shaped workpieces by means of digital print heads. In this respect, the print heads operate according to a jet printing method known from ink-jet printers. For this purpose, they are arranged in one or more rows transversely to the feed direction of the workpiece and at a relatively short distance above said workpiece (cf. DE 10 2009 044 802 A1 and EP 2 218 520 A2).
However, a problem which arises during the operation of these known digital applicator devices is the risk of collisions which can occur at the digital print heads when there are unevennesses in the board-shaped workpieces, for example during cupping of the boards and/or if there are foreign bodies on the boards.